1. Field of the Invention
The present invention relates to a memory device capable of recording information and a storage apparatus using such a memory device.
2. Description of the Related Art
In information equipment such as computers, a high-density DRAM (dynamic RAM (random-access memory)) which can be operated at high speed is widely used as a random-access memory.
However, since the DRAM is complex in manufacturing process as compared with ordinary logic circuit (large scale integrated circuit (LSI circuit)) and a signal processing circuit, it is unavoidable that the manufacturing cost of the DRAM increases.
Also, the DRAM is a volatile memory of which information is lost when de-energized so that refresh operations should be made frequently, that is, written information (data) should be read out, amplified and again written in the DRAM.
For this reason, a FeRAM (ferroelectric memory) and an MRAM (magnetic memory device) have been so far proposed as a nonvolatile memory of which information can be prevented from being lost even when de-energized.
It becomes possible for these memories to maintain written information during a long period of time without supply of power.
Since these memories are nonvolatile memories, the refresh operations need not be made and it is expected that power consumption will be decreased.
However, the above-mentioned nonvolatile memories become difficult to maintain characteristics of memory devices as the memory device comprising each memory cell is being reduced in size.
As a consequently, it is difficult to reduce the device in size up to the limit of a design rule and up to the limit of a manufacturing process.
Accordingly, a memory device of a new type has been proposed as a memory which is suitable for being reduced in size.
This type of memory device has a structure in which an ion conductor containing some metal is sandwiched between two electrodes.
Since any one of the two electrodes contain a metal contained in the ion conductor, when a voltage is applied to the two electrodes, the metal contained in the electrode is diffused into the ion conductor as ions, whereby electric characteristics such as a resistance value or a capacitance of the ion conductor are changed.
It is possible to construct the memory device by using this characteristic (for example, see cited patent reference 1 and cited non-patent reference 1).
To be concrete, the ion conductor is made of solid solution of chalcogenide and metal. More specifically, the ion conductor is made of solid solution of AsS, GeS, GeSe with Ag, Cu, Zn. Any one of the two electrodes contains Ag, Cu, Zn Z (see the above-described cited patent reference 1).
Further, various kinds of nonvolatile memories using crystal oxide materials have been proposed. For example, in a device having a structure in which a Cr-doped SrZrO3 crystal material is sandwiched by a lower electrode made of SrRuO3 or Pt and an upper electrode made of Au or Pt, there has been reported a memory in which a resistance is reversibly changed with application of voltages with different polarities (see cited non-patent reference 2). However, the details of its principle and so on are unknown.
[Cited patent reference 1]: Official gazette of Japanese laid-open patent application No. 2002-536840
[Cited non-patent reference 1]: Nikkei Electronics, Jan. 20, 2003 (p. 104)
[Cited non-patent reference 2]: A. Beck et al., Appl. Phys. Lett., 77 (2000), p. 139
However, the above-mentioned memory device having the structure in which any one of the upper electrode or the lower electrode contains Ag, Cu, Zn and in which the GeSe amorphous chalcogenide material is sandwiched by these electrodes encounters with the problem in which the aforementioned ion conductor whose resistance is changes is promoted to be crystallized by causes such as the rise of temperature in the manufacturing process, the rise of temperature generated by Joule heat of a recording current, a heat load over a long period of time in which data is stored during a long period of time so that the ion conductor is wholly or partly crystallized, thereby changing the original electric characteristics such as the change of the resistance value of the memory device and the change of recording/erasing operation voltage.
Then, when the crystal material is used as the recording material between the upper and lower electrodes, for example, as compared with the case in which the amorphous material is used as the above-mentioned recording material between the upper and lower electrodes, this memory device encounters with many problems and it is difficult to mass-produce this memory device inexpensively.
Also, in order to obtain crystallinity of excellent quality, a high-temperature treatment at 700° C. should be carried out, and there arises a problem in which characteristics of an MOS transistor that has been formed in advance are deteriorated by heat.
Further, in order to enable the crystal to grow, the material of the underlayer is limited and hence a single crystal material should be used.